1. Field of the Invention
This invention relates to a molecular beam epitaxy (MBE) method for the epitaxial growth of Group III nitride semiconductor materials, such as GaN.
2. Description of the Related Art
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decompoiltion to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the GaN system, an ultra-high vacuum (URV) environment, typically around 1xc3x9710xe2x88x923 Pa, is used. Ammonia or another nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing gallium and, possibly, indium and/or aluminium are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
GaN has a lattice constant of around 0.45 nm. There is a lack of suitable substrates that are lattice-matched to GaN, so GaN is generally grown onto either a sapphire substrate or a silicon carbide substrate. Because of the lattice mis-match between GaN and sapphire or silicon carbide, it is necessary to provide a thin initial nucleation layer on the substrate in order to grow high quality GaNs on sapphire or silicon carbide.
Akasaki and Amano report, in xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d Vol. 36, pp5393-5408 (1997), that a thin AlN layer, deposited at a low growth temperature, can be used as a nucleation layer to promote the growth of a GaN layer by metal organic chemical vapour deposition (MOCVD) process on a sapphire or silicon carbide substrate.
U.S. Pat. No. 5,290,393 discloses the use of a GaN nucleation layer, deposited at a low growth temperature, for promoting the growth of a GaN layer using MOCVD.
The above prior art documents do not discuss the effect of thermal treatment of the nucleation layer on the properties of the subsequent GaN layer. Lin et al in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 68, pp3758-3760 (1996) and Sugiura et al in xe2x80x9cJournal of Applied Physicsxe2x80x9d Vol. 82, p4877-4882 (1997) report that controlled annealing of the nucleation layer can improve the quality of the subsequent GaN layer. Both of these studies on the effect of annealing the nucleation layer were carried out for GaN layers grown by MOCVD.
At present, the majority of growth of high quality GaN layers is carried out using the MOCVD process. The MOCVD process allows good control of the growth of the nucleation layer and of the annealing of the nucleation layer. Furthermore, the MOCVD process allows growth to occur at a V/III molar ratio well in excess of 1000:1. (The V/Ill molar ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III molar ratio is preferable, since this allows a higher substrate temperature to be used leading to a higher quality GaN layer.)
At present, growing high quality GaN layers by MBE is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process; it is difficult to obtain a V/III molar ratio of 10:1 or greater. The two commonly used sources of nitrogen in the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
U.S. Pat. No. 5,385, 862and WO 92/16966 disclose a method of growing a single crystal GaN film on a sapphire substrate using MBE. In this method, the nitrogen used in the growth process is activated nitrogen obtained by exciting molecular nitrogen using a plasma source. In this method, the growth of the initial nucleation layer is restricted to a growth temperature of 400xc2x0 C. or lower, and the growth of the subsequent GaN layer is restricted to a growth temperature of lower than 900xc2x0 C. This patent discloses a step of crystallising the GaN nucleation layer, but specifies no time period for this step. GaN layers grown by this method have electron mobilities at room temperature of less than 100 cm2Vxe2x88x921Sxe2x88x921.
A method of growing GaN by MBE in which ammonia is used as the nitrogen source is reported by Yang et al in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol 67, pp1686-1688 (1995). In this prior art method, a GaN nucleation layer is grown using nitrogen obtained from plasma excited molecular nitrogen, and the nucleation layer is not annealed. The subsequent GaN semiconductor layer is grown at the low growth temperature of 750xc2x0 C. due to the low V/III molar ratio of the growth process. This method produces GaN layers having a room temperature mobility of less than 100 cm2Vxe2x88x921sxe2x88x921.
Further reports on the grown of GaN layers using MBE, with ammonia as the nitrogen source, have been made by Grandjean et al in (1) xe2x80x9cJournal of Applied Physicsxe2x80x9d Vol 83, pp1379-1383 (1998) and (2) xe2x80x9cApplied Physics Lettersxe2x80x9d Vol 71, p240-242 (1997). Both of these documents report methods in which a GaN nucleation layer is used. The nucleation layer is annealed, although at the low temperature of 900xc2x0 C. owing to the low quantity of ammonia present during the annealing step. The subsequently grown GaN layer is grown at a low growth temperature of 830xc2x0 C. owing to the low V/III ratio obtained in the growth process. Grandjean et al (1) report a V/III ratio (atomic nitrogen to gallium) of 3-5:1 (equivalent to a molar ratio of about 60-100:1), whereas Grandjean et al (2) do not give a value for the V/III ratio. The methods reported in these two documents produce GaN layers having a room temperature electron mobility of less than 150 cm2Vxe2x88x921sxe2x88x921.
EP-A-0 607 435 discloses a method of growing a nitride semiconductor material by MBE. The method comprises growing a thin layer of an oriented polycrystalline nitride semiconductor material on a substrate, and subsequently growing a single crystal nitride semiconductor layer on the thin polycrystalline layer. The polycrystalline layer is not annealed, so that the thin layer will retain its polycrystalline structure when the single crystal layer is grown.
EP-A-0 497 350 relates to the growth of a GaN layer on a sapphire substrate by an MOCVD process. A GaAlN buffer layer is grown on the substrate, and a GaN layer is grown on the buffer layer. This document investigates how changes in the composition of the buffer layer affect the quality of the epitaxial layer.
It can therefore be seen that it is desirable to make possible a method for growing high mobility GaN layers using the MBE process. In order to do this, it is necessary to increase the growth temperatures of the GaN layer. The growth temperature is important in order to achieve polycrystalline material and good surface coverage. The annealing temperature is important in order to crystallise and flatten the nucleation layer.
A first aspect of the present invention provides a method of growing a nitride semiconductor layer by molecular beam epitaxy, the method comprising the steps of:
(a) disposing a substrate in a vacuum chamber;
(b) growing a nucleation layer of GaxAl1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) on the substrate by molecular beam epitaxy;
(c) annealing the GaxAl1xe2x88x92xN layers; and
(d) growing a nitride semiconductor layer on the GaxAl1xe2x88x92xN layer by molecular beam epitaxy at a V/III molar ratio within the vacuum chamber of 100:1 or greater; wherein the method further comprises supplying ammonia gas to the vacuum chamber in step (b) and step (d).
Because the growth of the GaN layer is carried out at a V/III molar ratio within the vacuum chamber of 100:1 or greater in the present invention, the growth process can be carried out at a higher temperature. In consequence, the present invention produces a higher quality nitride semiconductor layer than do the prior art methods. The present invention can produce, for example a GaN layer having an electron mobility at room temperature greater than 250 cm2Vxe2x88x921sxe2x88x921. This mobility compares favourably with the mobilities obtained by the MOCVD method. Moreover, supplying ammonia gas to the vacuum chamber in step (b) and step (d) is a convenient way of providing the nitrogen required for the MBE growth process, and eliminates the need to generate a nitrogen or ammonia plasma to provide activated nitrogen.
Step (d) may comprise growing the nitride layer at a V/III molar ratio within the vacuum chamber of 500:1 or greater or at a V/III molar ratio within the vacuum chamber of 1000:1 or greater. Step (d) may comprise growing the nitride layer at a V/III molar ratio within the vacuum chamber in the range 100:1-10,000:1.
Step (b) may comprise growing the GaxAl1xe2x88x92xN layer at a V/III molar ratio within the vacuum chamber of at least 100:1. It may comprise growing the GaxAl1xe2x88x92xN layer at a V/III molar ratio within the vacuum chamber in the range 100:1-10,000:1.
The thickness of the GaxAl1xe2x88x92xN layer grown in step (b) may be within the range 5-200 nm.
The substrate temperature during step (b) may be within the range 400-600xc2x0 C.
Step (a) may comprise supplying ammonia gas to the vacuum chamber during the step of annealing of the GaxAl1xe2x88x92xN layer.
The substrate temperature during the step of annealing of the GaxAl1xe2x88x92xN layer may be 950-1050xc2x0 C.
The duration of the step of annealing the GaxAl1xe2x88x92xN layer may be between 0 and 90 minutes.
The duration of the step of annealing the GaxAl1xe2x88x92xN layer may be approximately 45 minutes and the thickness of the nucleation layer may be 20 nm. Alternatively, the duration of the step of annealing the GaxAl1xe2x88x92xN layer may be approximately 15 minutes and the thickness of the nucleation layer may be 70 nm.
The substrate temperature during step (d) may be within the range 900-1050xc2x0 C.
A second aspect of the present invention provides a nitride semiconductor layer grown by a method defined above.
A third aspect of the present invention provides a semiconductor device comprising a nitride semiconductor layer defined above.